This invention is a surgical device for producing a circular, interlamellar channel within the corneal stroma. An intracorneal ring can be implanted into this channel at the corneal periphery modifying the corneal curvature while sparing the important central optical zone of the cornea. This channel is formed by the sequential use of two separate instruments. The first instrument, the channel-guide dissector, is a circular dissecting instrument which is a split ring dissector with the dissecting end having a blunt tip and the other end connected to a handle. The important aspect of this channel-guide dissector is that it has a relatively narrow width which results in production of a lamellar corneal channel that is typically too narrow for the insertion of a typical intracorneal ring for myopic adjustment.
However, the narrow channel produced serves as a guide for the introduction of the second instrument, also a lamellar dissecting instrument. The unique feature of this instrument is the design of the leading tip of this device. This leading tip is similar in shape to the tip of the first instrument and thus serves to guide the second instrument into the previously formed channel. Behind the leading tip is a wider flat blade that is gradually swept posteriorly, similar to a wing. The leading portion of this wing acts as the dissecting portion of the second instrument, also referred to as the winged dissector.
Ametropia, an undesirable refractive state of the eye, has three main subdivisions: myopia, hyperopia, and astigmatism. In myopia, by far the most common type of ametropia, the parallel light rays 20 which enter the eye as shown in FIG. 1 come to a focus F1 in front of the retina 24 as shown in FIG. 2. In hyperopia, the rays of light 20 come to a focus F2 behind the retina 24 as shown in FIG. 2. When the rays of light converge to not one, but several foci, it is referred to as astigmatism, in which condition the various foci may all lie before the retina; all lie behind the retina; or partly before the partly behind the retina.
Controversy has always surrounded the use of surgical procedures to correct refractive errors of the eye. Because of the risks inherent in surgical intervention, some has argued that no refractive error correctable by spectacles or contact lenses warrants such procedures. Until an ideal refractive procedure is developed, disagreement will persist among ophthalmologists as to which eyes and which patients are appropriate for refractive surgery. The ideal keratorefractive procedure for the correction of myopia should permit all the advantages of eyeglasses or contact lenses, namely, being able to correct a wide range of refractive errors, generating a high degree of predictability and stability such that &lt;95% of patients achieve 20/20 uncorrected acuity with long-term stability, allowing reversibility or adjustability in the event that the refractive state of the eye changes, being extremely safe with minimal risk of adverse effects on the quality of vision and being cost effective. The refractive procedure should also have a favorable learning curve. If a technique permits only a small percentage of ophthalmologists to perform and achieve excellent visual results, it is a seriously deficient procedure.
For over a century, ophthalmologists have searched for a surgical method to permanently correct refractive errors. Over 15 different techniques have been developed and considerable experience gained in both animal and human models. Oftentimes a given refractive surgical technique has unsolved problems such as poor predictability, unstable refractive outcomes, adverse effects on the quality of vision, lack of adjustability and irreversibility. Poor predictability remains the largest unsolved problem in refractive corneal surgery. The main factors that contribute to poor predictability are: ) 1 variations and inaccuracies inherent with manual surgical techniques and 2) the variable would healing response to the surgery. Photorefractive keratectomy offers the possibility of reducing the surgical variability of the procedure. However, the variable corneal wound healing response affects the results of photorefractive keratectomy manifesting as regression of refractive effect which can be up to several diopters in amplitude.
For years it has been thought that refractive surgery with intracorneal implants could be used in the correction of myopia or hyperopia. Early techniques included lamellar removal or addition of natural corneal stromal tissue. These techniques required the use of a microkeratome to remove a portion of the cornea followed by lathing of either patient's or donor's removed cornea. The equipment is complex, the surgical techniques difficult, and most disappointingly, the results quite variable. The current trend in keratorefractive surgery has been toward techniques that are less traumatic to the cornea, that minimally stimulate the wound healing response, and behave in a more predicable fashion. the use of alloplastic intracorneal lenses to correct the refractive state of the eye, first proposed in 1949 by Jose Barraquer, have been plagued with problems of bicompatibility, permeabilty of nutrients and oxygen, etc.
More recent techniques have focused on minimizing the effects of the wound healing response by avoiding the central cornea. There have been multiple attempts to alter the central corneal curvature by surgically manipulating the peripheral cornea. These reply upon mechanisms first elucidated by J. Barraquer. Since 1964, "it has been demonstrated that to correct myopia, thickness must be subtracted from the center of the cornea or increased in its periphery, and that to correct hyperopia, thickness must be added to the center of the cornea or increased in its periphery." D. S. Zhivotosvskii, in USSR patent No. 3,887,846, describes an alloplastic, flat, geometrically regular, annular ring for intracorneal implantation of a diameter that does not exceed the diameter of the pupil. Refractive correction is accomplished primarily by making the radius of curvature of the surface of the ring larger than the radius of curvature of the surface of the recipient's cornea in order to achieve flattening of the central area of the cornea. The principle is simply that either insertion of an intracorneal ring in the corneal periphery or injection of gels is a preformed peripheral circumferential channel will induce flattening of the center of the cornea. Qualitatively, the same effect may be induced by a rigid ring in a gel. the addition of matter in the corneal periphery will induce an outward bulging of the corneal surfaces around the implant, thus incorporating excess corneal arc length. Consequently, less of the corneal arc will be available for covering the central cornea, which therefore must flatten.
Intracorneal rings have several advantages over photorefractive keratectomy (PRK) including leaving the central cornea intact, reversibility, rapid and stable effect and minimal wound healing responses. However, predictability is still an issue and there is no simple way of adjusting the refractive outcome, aside from explanting the intracorneal ring and replacing with another ring of different size. Methods of adjusting an intracorneal ring after the ring has been implanted in a simple, minimally invasive, predictable fashion have been described. This method is described more fully by J. Lee in U.S. Pat. No. 5,733,334, which disclosure is incorporated herein by reference.
If the adjustable corneal ring is shown to be truly adjustable in a discrete fashion, then it would meet many of the criteria for an ideal refractive procedure. Safety of the surgical procedure is still an issue. Current methods of producing a peripheral intrastromal lamellar channel within the cornea rely upon certain biomechanical properties of the cornea. At a central corneal thickness of 0.50 mm, the corneal elastic stiffness is approximately 49 kPa. However, the cornea shows very little resistance to shear deformation in the plane tangential to the surface. In shearing experiments, the shear stiffness has been found to be approximately 2 kPa. Thus a blunted circular blade can travel in a lamellar plane, separating the layers to form an interlamellar channel. In contrast, a sharp circular blade will not stay in an interlamellar channel and can more easily completely penetrate the cornea entering the anterior chamber. This is not a desirable outcome.
Methods for producing an interlamellar channel have been described. Simon in U.S. Pat. No. 5,090,955 describes a flat corkscrew delaminator which is used to carve a circular canal between the two corneal lamellae in which a gel such as a silicon gel is subsequently injected. The corkscrew delaminator consists of a flat wire about 1 mm or less in width and its edges are blunt or rounded as is its end.
A. E. Reynolds in U.S. Pat. No. 4,452,235 also describes a split-ring shaped dissecting member designed to produce a peripheral interlamellar corneal channel to permit implantation of an interacorneal ring.
There is, however, still a risk of corneal perforation into the anterior chamber with lamellar dissecting devices even with a blunted tip. Thus, there remains a need for a method to decrease the risk of perforating the cornea while producing a lamellar channel, especially in light of the fact that refractive surgery procedures today are almost universally an elective procedure.
There are several problems with currently used corneal delaminators. One problem is that if the dissecting tip is too sharp the delaminator or dissector may not stay in the interlamellar space but cut across lamellae resulting in a channel with uneven depth or at worst, perforate through Descemet's membrane into the anterior chamber of the eye. The blade may drift more anterior as it follows the arcuate path and perforate anteriorly though Bowman's membrane. This problem was overcome by the discovery that a comparatively blunt but rounded device would provide an intrastromal channel with a fairly constant depth without crossing interlamellar layers. However, blunted dissectors have their share of problems as will be described.
Another problem is centration of the channel around the apex of the cornea. Centration is important in the final refractive result after intracorneal ring placement. Proper centration is required to avoid postoperative complications, like irregular astigmatism and glare, that may interfere with visual function. Because the tip of the delaminator is made blunt, there is resistance to the passage of the delaminator which can cause distortion of the cornea during the dissection and result in decentration of the produced circular channel. B. Loomis in U.S. Pat. No. 5,403,335 describes a combination of vacuum chambers, insert rings, and ridges within a vacuum chamber to prevent this twisting of the corneal surface during insertion of the dissecting blade.
Using a vacuum suction ring stabilizes the cornea and is necessary in lamellar refractive procedures. When greater stabilization of the cornea is required, the vacuum on the suction ring must be increased with concomitant rise in the intraocular pressure. Strong suction maintains the cornea but also causes high intraocular pressure. Current dissectors require more stabilization because their dissecting blades are made blunt. The danger of corneal perforation is aggravated when the high intraocular pressure is followed by a sudden decrease in intraocular pressure in the event of corneal perforation. The damage caused by this sudden decrease in pressure can range from just the perforation itself to more serious consequences, such as damage to the iris or the lens or even an expulsive hemorrhage. A sudden decrease in intraocular pressure from a perforating injury may cause the intraocular contents to expulse. Thus a procedure that can be performed with minimal suction, requiring minimal corneal stabilization, even if the cornea undergoes perforation is less likely to cause severe anterior segment damage.
Lastly, as the channel is being formed by the rotational movement of a given circular delaminator, the friction forces add resistance to the forward movement of the tip of the circular delaminator and the initial incision (1) perpendicular to the corneal surface is gaped open. Healing in this area can be complicated by tears from the shearing forces. One solution was to partially complete the channel by using a dissector in one direction followed by completion of the interlamellar channel by using another dissector which is rotated in the opposite direction. Complete rotation into the cornea of a dissector involves more resistance to forward direction and complications of tearing the initial incision than a partial rotation.